Nonaqueous secondary battery and method of using the same

ABSTRACT

A nonaqueous secondary battery containing a positive electrode having a positive electrode mixture layer, a negative electrode, and a nonaqueous electrolyte, in which the positive electrode comprises, as active materials, three or more lithium-containing transition metal oxides having different average particle sizes, those transition metal oxides comprise a transition metal element, the transition metal element of the transition metal oxide having the smallest average particle size is partially substituted with a metal element other than the transition metal element, and the density of the positive electrode mixture layer is t 3.8 g/cm 3  or more.

FILED OF THE INVENTION

The present invention relates to a nonaqueous secondary battery and amethod of using the same.

RELATED ART

In recent years, the secondary battery is an indispensable, importantdevice as a power source of a personal computer or a cellular phone, ora power source for an electric vehicle or an electric power storage.

In particular, in applications for a mobile communication device such asa portable computer and a personal digital assistant, the battery isrequired to be made smaller and to trim weight. Under the currentcircumstances, however, the system of the battery is not easily madecompact or lightweight, since an electric power consumed by a back lightof a liquid crystal display panel or consumed to control the drawing ofgraphics is large, or the capacity of a secondary battery is notsufficiently large. In particular, a personal computer is progressivelymulti-functionalized by mounting a digital versatile disc (DVD) driveand so on. Thus, the power consumption thereof tends to increase. Forthis reason, it is highly required to increase the electric capacity ofa secondary battery, in particular, the discharge capacity, when thevoltage of a single battery is 3.3 V or higher.

Attention is paid to electric vehicles, which discharge no exhaust gasand make less noise in association with the increase of globalenvironmental problems. Recently, hybrid electric vehicles (HEV), whichadopt a system of storing regenerative energy generated at the time ofbraking in a battery and making effective use of the energy, or using anelectric energy stored in a battery at the time of engine starting toincrease the efficiency of the engine system, have gained popularity.However, since the electric capacity of the currently used battery issmall, a plurality of batteries should be used to generate a sufficientvoltage. For this reason, problems such that a space in the vehicleshould be made smaller and that the stability of the vehicle bodydeteriorates arise.

Among secondary batteries, a lithium secondary battery using anonaqueous electrolyte attracts attention, since it generates a highvoltage, has a lightweight and is expected to achieve a high energydensity. In particular, a lithium secondary battery disclosed inJP-A-55-136131, in which a lithium-containing transition metal oxide,for example, LiCoO₂, is used as a positive electrode active material,and metal lithium is used as a negative electrode active material, isexpected to attain a high energy density, since it has an electromotiveforce of 4 V or higher.

However, at present, in the case of a LiCoO₂ based secondary batterywhich uses LiCoO₂ as a positive electrode active material and acarbonaceous material such as graphite as a negative electrode activematerial, a charge final voltage thereof is usually 4.2 V or less.According to this charging condition, the charge capacity is only about60% of the theoretical capacity of LiCoO₂. The electric capacity may beincreased by increasing the charge final voltage to higher than 4.2 V.However, with the increase of the charge capacity, the crystallinestructure of LiCoO₂ decays so that the charge-discharge cycle life maybe shortened, or the crystalline structure of LiCoO₂ may bedestabilized. Accordingly, the thermal stability of the batterydeteriorates.

To solve such a problem, many attempts have been made to add a differentmetal element to LiCoO₂ (cf. JP-A-4-171659, JP-A-3-201368, JP-A-7-176302and JP-A-2001-167763).

In addition, attempts have been made to use a battery in a high-voltagerange of 4.2 V or higher (cf. JP-A-2004-296098, JP-A-2001-176511 andJP-A-2002-270238).

In years to come, a secondary battery will be required to have a highercapacity and also better reliability including higher safety than theconventional batteries. In general, the battery capacity can be greatlyimproved by raising the content of an active material in electrodes orby increasing an electrode density, in particular, the density of apositive electrode mixture layer. However, according to suchcapacity-increasing measures, the safety of the battery graduallydecreases.

Accordingly, in order to meet requirements for the increase of theelectric capacity, it is highly desired to provide a battery which usesa material that generates a higher electromotive force (voltage range)than LiCoO₂ and has a stable crystalline structure capable of beingstably and reversibly charged and discharged, and which furthersatisfies reliability such that the safety of the conventional batteriescan be maintained and the battery does not expand when the density ofthe positive electrode mixture layer is increased.

When the discharge final voltage of a conventional battery comprisingLiCoO₂ as a positive electrode active material is made higher than 3.2V, the battery cannot be completely discharged since the voltage in thefinal stage of the discharge significantly falls. Thus, an electricquantity efficiency of discharge relative to charging remarkablydecreases. Since the complete discharge cannot be attained, thecrystalline structure of LiCoO₂ easily decays, and thus thecharge-discharge cycle life is shortened. This phenomenon remarkablyappears in the above-mentioned high voltage range.

Under a charging condition that the final voltage at full charging isset to 4.2 V or higher in the conventional battery, apart fromshortening of the charge-discharge cycle life or the decrease of thethermal stability caused by the decay of the crystalline structure ofthe positive electrode active material, the electrolytic solution (asolvent) is oxidatively decomposed due to the increase of the activesites in the positive electrode active material, whereby a passivationfilm is formed on the surface of the positive electrode and thus theinternal resistance of the battery increases so that the loadcharacteristic may deteriorate.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a nonaqueous secondarybattery having a high voltage, a high capacity and high reliabilityincluding high safety, and a method of using the same.

Accordingly, the present invention provides a nonaqueous secondarybattery comprising: a positive electrode having a positive electrodemixture layer, a negative electrode, and a nonaqueous electrolyte,wherein the positive electrode comprises, as active materials, at leastthree lithium-containing transition metal oxides having differentaverage particle sizes, the lithium-containing transition metal oxidescomprise at least one transition metal element M¹ selected from thegroup consisting of Co, Ni and Mn, the transition metal element M¹ ofthe lithium-containing transition metal oxide having the smallestaverage particle size is partially substituted with a metal element M²other than the transition metal element M¹, and the density of thepositive electrode mixture layer is at least 3.8 g/cm³.

The “average particle size” of the lithium-containing transition metaloxides used herein means a 50% diameter value (d₅₀), that is, an mediandiameter, read from an integral fraction curve based on volumes, whichis obtained by integrating the volumes of the particles from a smallerparticle size measured by a MICROTRAC particle size analyzer (HRA 9320available from NIKKISO Co., Ltd.).

The present invention also provides a method of using a nonaqueoussecondary battery according to the present invention comprising the stepof charging the battery so that a positive electrode voltage is in arange of 4.35 to 4.6 V with reference to the potential of lithium whenthe battery is fully charged.

For example, in the method of using the nonaqueous secondary batteryaccording to the present invention, when the nonaqueous secondarybattery of the present invention comprises a graphite negativeelectrode, namely, a negative electrode containing graphite as anegative electrode active material, which has a voltage of 0.1 V withreference to the lithium potential when the battery is fully charged,charging the battery up to a battery voltage of 4.45 V or higher isregarded as charging the battery so as to substantially attain apositive electrode voltage of 4.35 V or higher.

In the method of using the nonaqueous secondary battery according to thepresent invention, the term “fully charging (charged)” means chargingunder the following conditions: the battery is charged at a constantcurrent of 0.2 C up to a predetermined voltage and subsequently thebattery is charged at a predetermined constant voltage, provided thatthe total time of the constant current charging and the constant voltagecharging is set to 8 hours.

Accordingly, the nonaqueous secondary battery of the present inventionhas a high voltage, a high capacity and high reliability including highsafety.

According to the using method of the present invention, the nonaqueoussecondary battery of the present invention can be used in applicationswhich require larger power output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B schematically show one example of the nonaqueoussecondary battery of the present invention. FIG. 1A is a plan viewthereof and FIG. 1B is a partial vertical section thereof.

FIG. 2 shows a perspective view of the nonaqueous secondary batteryillustrated in FIGS. 1A and 1B.

DETAILED DESCRIPTION OF THE INVENTION

The positive electrode used in the nonaqueous secondary battery of thepresent invention comprises, as the active material, the mixture of atleast three lithium-containing transition metal oxides having differentaverage particle sizes, each of which comprises at least one transitionmetal selected from the group consisting of Co, Ni and Mn as atransition metal element M¹. Therefore, gaps between the particles ofone lithium-containing transition metal oxide having relatively largeparticle size are filled with the particles of other transition metaloxide having a smaller particle size than the one transition metal oxidein the positive electrode mixture layer. The gaps remaining unfilledeven in this manner can be filled with the lithium-containing transitionmetal oxide having an even smaller particle size. Accordingly, thedensity of the positive electrode mixture layer is increased and, inturn, the capacity of the battery is increased.

The mixture of “at least three lithium-containing transition metaloxides having different average particle sizes” has five or moreinflection points in a particle size distribution curve of the mixture.The particle size distribution curve may have three or more peaks or ashoulder in one or more peaks. In the case of such a particle sizedistribution curve, firstly, a conventional peak-separating method isapplied to separate a distribution of particles having a larger particlesize and that of particles having a smaller particle size. Subsequently,from the particle sizes and the integrated volume, the average particlesize (d₅₀) of each of the lithium-containing transition metal oxides andthe mixing ratio between them can be calculated.

In the case of using, for example, three transition metal oxides havingdifferent average particle sizes as the lithium-containing transitionmetal oxides in the positive electrode, the average particle size of thelithium-containing transition metal oxide having the largest averageparticle size (hereinafter referred to as “positive electrode activematerial (A)”) is represented by A, that of the lithium-containingtransition metal oxide having the second largest average particle size(hereinafter referred to as “positive electrode active material (B)”) isexpressed by B, and that of the lithium-containing transition metaloxide having the smallest average particle size (hereinafter referred toas the “positive electrode active material (C)”) is expressed by C.Then, the ratio of B to A (i.e., B/A) is preferably from 0.2 to 0.3, andthe ratio of C to B (i.e., C/B) is preferably from 0.1 to 0.25. When theaverage particle sizes of the three positive electrode active materials(A), (B) and (C) satisfy these relationships, the density of thepositive electrode mixture layer can be easily increased.

In the case of using 4 or more oxides having different average particlesize as the lithium-containing transition metal oxides in the positiveelectrode, the average particle size of each of the lithium-containingtransition metal oxides other than the positive electrode activematerial (A) having the largest average particle size and the positiveelectrode active material (C) having the smallest average particle sizepreferably satisfies the same relationships of the average particle sizeB of the positive electrode active material (B) with the averageparticle size A of the positive electrode active material (A) and theaverage particle size C of the positive electrode active material (C) asexplained above.

The positive electrode active material (A) preferably has an averageparticle size of 5 μm or more, more preferably 8 μm or more,particularly preferably 11 μm or more. When the average particle size ofthe positive electrode active material (A) is too small, the density ofthe positive electrode mixture layer may hardly be increased. When theaverage particle size is too large, the battery characteristic tends todecrease. Thus, the average particle size is preferably 25 μm or less,more preferably 20 μm or less, particularly preferably 18 μm or less.

The positive electrode active material (B) preferably has an averageparticle size of 10 μm or less, more preferably 7 μm or less,particularly preferably 5 μm or less. When the average particle size ofthe positive electrode active material (B) is too large, the positiveelectrode active material (B) does not easily fill the gaps between theparticles of the lithium-containing transition metal oxide having arelatively large particle size in the positive electrode mixture layer,so that the density of this layer may hardly be increased. When theaverage particle size is too small, the number of voids increases sothat the density of the positive electrode mixture layer may not beincreased. Thus, the average particle size of the positive electrodeactive material (B) is preferably 2 μm or more, more preferably 3 μm ormore, particularly preferably 4 μm or more.

The positive electrode active material (C) preferably has an averageparticle size of 4 μm or less, more preferably 3 μm or less,particularly preferably 2 μm or less. When the average particle size ofthe positive electrode active material (C) is too large, the positiveelectrode active material (C) does not fill the gaps between theparticles of the positive electrode active material (A) and/or thepositive electrode active material (B) in the positive electrode mixturelayer, so that the density of this layer may hardly be increased. Whenthe average particle size of the positive electrode active material (C)is too small, the safety of the battery may be adversely affected. Thus,the average particle size of the positive electrode active material (C)is preferably 0.5 μm or more, more preferably 0.8 μm or more,particularly preferably 1 μm or more.

The positive electrode active materials according to the presentinvention may contain three lithium-containing transition metal oxideshaving different average particle sizes, for example, the positiveelectrode active materials (A), (B) and (C) as descried above, while thepositive electrode active materials may contain four or more, forexample, four, five or six lithium-containing transition metal oxideshaving different average particle sizes, as described above.

The content of the positive electrode active material (C) having thesmallest average particle size in the lithium-containing transitionmetal oxides contained in the positive electrode is preferably 1% byweight or more, more preferably 2% by weight or more, particularlypreferably 3% by weight or more. When the positive electrode activematerial (C) is contained in an amount of the above-mentioned range, thegaps between the particles of the lithium-containing transition metaloxide having a relatively large particle size, for example, the positiveelectrode active materials (A) and (B), are easily filled therewith sothat the density of the positive electrode mixture layer is increased.When the content of the positive electrode active material (C) is toolarge, the density of the positive electrode mixture layer is hardly beincreased. Thus, the content of the positive electrode active material(C) is preferably 10% by weight or less, more preferably 7% by weight orless, particularly preferably 5% by weight or less.

When the lithium-containing transition metal oxides contained in thepositive electrode are only three oxides having different averageparticle sizes, for example, only the positive electrode activematerials (A), (B) and (C), the content of the positive electrode activematerial (B) having a medium average particle size is preferably 8% byweight or more, more preferably 11% by weight or more, particularlypreferably 13% by weight or more of the oxides. When the positiveelectrode active material (B) is contained in an amount in this range,the gaps between the particles of the lithium-containing transitionmetal oxide having a relatively large average particle size such as thepositive electrode active material (A) are easily filled therewith sothat the density of the positive electrode mixture layer is increased.When the content of the positive electrode active material (B) is toolarge, the density of the positive electrode mixture layer is hardlyincreased. Thus, the content of the positive electrode active material(B) is preferably 25% by weight or less, more preferably 22% by weightor less, particularly preferably 20% by weight or less.

Accordingly, when the lithium-containing transition metal oxidescontained in the positive electrode are only the positive electrodeactive materials (A), (B) and (C), for example, the content of thepositive electrode active material (A) in all the lithium-containingtransition metal oxides is preferably 65% by weight or more, morepreferably 67% by weight or more, particularly preferably 72% by weightor more. On the other hand, the content is preferably 90% by weight orless, more preferably 85% by weight or less, particularly preferably 80%by weight or less.

Among the lithium-containing transition metal oxides contained in thepositive electrode, the positive electrode active material (C) havingthe smallest average particle size has the above-mentioned averageparticle size. Such a lithium-containing transition metal oxide having arelatively small particle size has low stability, for example, in astate that the battery is charged at a high voltage, so that the oxidemay damage the reliability including the safety of the battery.

Thus, the present invention uses, as at least the positive electrodeactive material (C), which is the lithium-containing transition metaloxide having the smallest average particle size, a lithium-containingtransition metal oxide the transition metal element M¹ of which ispartially substituted with a metal element M² other than the element M¹.Since the lithium-containing transition metal oxide the transition metalelement M¹ of which is partially substituted with the other metalelement M² has the improved stability, in particular, the stability in astate that the battery is charged at a high voltage. Accordingly, thereliability including the safety of the battery can be improved.Furthermore, because of the improved stability of the lithium-containingtransition metal oxide the transition metal element M¹ of which ispartially substituted with the other metal element M², the decay of theactive material (C) is suppressed, when the charge-discharge cycles arerepeated. Thus, the use of such a positive electrode active material (C)increases the charge-discharge cycle characteristics of the battery.

The following method can confirm whether the transition metal element M¹of at least the lithium-containing transition metal oxide having thesmallest average particle size is partially substituted with the othermetal element M²: The particle size distribution curve of the positiveelectrode active material to be used in the positive electrode isobtained; peaks appearing in the curve are subjected to theabove-described peak separation; and the particles belonging to the peakof the particles having the smallest particle size are analyzed with anelectron probe micro-analyzer (EPMA) or the like to confirm whether theycontain the metal element M².

In the present invention, it is sufficient that the positive electrodeactive material (C) having the smallest average particle size is alithium-containing transition metal oxide the transition metal elementM¹ of which is partially substituted with the other metal element M². Inaddition, it is preferable that at least one of the lithium-containingtransition metal oxides other than the positive electrode activematerial (C), for example, the positive electrode active material (A)having the largest average particle size and/or the positive electrodeactive material (B) having an average particle size between those of thepositive electrode active materials (A) and (C) may also be alithium-containing transition metal oxide the transition metal elementM¹ of which is partially substituted with the other metal element M². Insuch a case, the stability, in particular, the stability in a state thatthe battery is charged at a high voltage, of the oxides other than thematerial (C) is improved, so that the charge-discharge cyclecharacteristic of the battery, and the reliability including the safetythereof can be further improved.

In the positive electrode active material (C), and the otherlithium-containing transition metal oxides such as the positiveelectrode active material (A) having the largest average particle size,and the positive electrode active material (B) having an averageparticle size between those of the positive electrode active materials(A) and (C), the metal element M² which substitutes a part of thetransition metal element M¹ is preferably at least one metal elementselected from the group consisting of Mg, Ti, Zr, Ge, Nb, Al and Sn,since these elements have a very high effect of improving the stabilityof lithium-containing transition metal oxides.

The positive electrode active material (C) is preferably alithium-containing transition metal oxide represented by the followingformula (1):Li_(x)M¹ _(y)M² _(z)M³ _(v)O₂  (1)wherein M¹ represents at least one transition metal element selectedfrom Co, Ni and Mn, M² represents at least one metal element selectedfrom the group consisting of Mg, Ti, Zr, Ge, Nb, Al and Sn, M³represents an element other than Liu, M¹ and M², and x, y, z and v arenumbers satisfying the following equations respectively: 0.97≦x<1.02,0.8≦y<1.02, 0.002≦z≦0.05, and 0≦v≦0.05.

Each of the lithium-containing transition metal oxides other than thepositive electrode active material (C) such as the positive electrodeactive material (A) or (B) is preferably a lithium-containing transitionmetal oxide represented by the following formula (2):Li_(a)M¹ _(b)M² _(c)M³ _(d)O₂  (2)wherein M¹, M² and M³ are the same as defined in the formula (1), and a,b, c and d are numbers satisfying the following equations respectively:0.97≦a<1.02, 0.8≦b<1.02, 0≦c≦0.02, and 0≦d≦0.02.

M¹, M² and M³ are selected from the same elements as in the formula (1),but the elements selected or the constituting element ratios selected inthe individual positive electrode active materials having differentaverage particle sizes may differ from each other. For example, in thepositive electrode active material (B), Mg, Ti and Al may be selected,while in the positive electrode active material (A), Mg and Ti may beselected. As explained in this example, however, among the elements M²,preferably at least one common element is selected, more preferably atleast two common elements are selected, and particularly preferably atleast three common elements are selected.

In the case of the positive electrode active material (A), “c” ispreferably 0.0002 or more, more preferably 0.001 or more, and it ispreferably less than 0.005, more preferably less than 0.0025, and “d” ispreferably 0.0002 or more, more preferably 0.001 or more and it ispreferably less than 0.005, more preferably less than 0.0025 for thefollowing reason: the particle size of the positive electrode activematerial (A) is relatively large; thus, when the amount of M² and thelike added to the material (A) is relatively small, advantageous effectscan be attained; but, when the amount is too small, the effects toimprove the charge-discharge cycle characteristic or the safety of thebattery may be insufficient, while when the amount is too large, theelectrical characteristics of the battery tends to decrease.

In each of the lithium-containing transition metal oxides used in thepresent invention, the transition metal element(s) thereof is/arepreferably mainly Co and/or Ni. For example, the total amount of Co andNi is preferably 50% by mole or more based on all the transition metalelements contained in the lithium-containing transition metal oxides.

Preferably, the proportion of Co in the lithium-containing transitionmetal oxide is higher, since the density of the positive electrodemixture layer can be made higher. In the formulae (1) and (2), theproportion of Co in the transition metal element M¹ is preferably 30% bymole or more, more preferably 65% by mole or more, particularlypreferably 95% by mole or more.

The values of x in the formula (1) and a in the formula (2) may vary asthe battery is charged or discharged. Nevertheless, when the battery isan as-produced one, x and a are each preferably 0.97, more preferably0.98 or more, particularly preferably 0.99 or more, while x and a areeach preferably less than 1.02, more preferably 1.01 or less,particularly preferably 1.00 or less.

The values of y in the formula (1) and b in the formula (2) are eachpreferably 0.98 or more, more preferably 0.98 or more, particularlypreferably 0.99 or more, and they are each preferably less than 1.02,more preferably less than 1.01, particularly preferably less than 1.0.

Each of the positive electrode active material (C) represented by theformula (1), and the lithium-containing transition metal oxides otherthan the positive electrode active material (C) which are represented bythe formula (2) preferably contains Mg as the element M², since thesafety of the battery is more effectively improved. From the viewpointof increasing the effect attained by the use of Mg as the element M²,the content Mg is preferably 0.1% by mole or more, more preferably 0.15%by mole or more, particularly preferably 0.2% by mole or more by mole ormore based on the content of whole M¹.

When the content of Mg in the positive electrode active material (C) orthe other lithium-containing transition metal oxides is too large, theload characteristic of the battery tends to decrease. Accordingly, thecontent of Mg is preferably less than 2% by mole, more preferably lessthan 1% by mole, particularly preferably less than 0.5% by mole based onthe content of M¹.

Each of the positive electrode active material (C) and the otherlithium-containing transition metal oxides preferably contains, as M²,at least one metal element selected from Mg, Ti, Zr, Ge, Nb, Al and Sn,in particular, Mg. More preferably, each of them contains Mg and also atleast one metal element selected from Ti, Zr, Ge and Nb. Mostpreferably, each of them further contains Al and/or Sn as the elementM². In these cases, the stability of those lithium-containing transitionmetal oxides is further improved in a state that the battery is chargedat a high voltage. When each of them contains, as the element M², atleast one metal element selected from Ti, Zr, Ge and Nb, the totalcontent of Ti, Zr, Ge and Nb is preferably 0.05% by mole or more, morepreferably 0.08% by mole or more, particularly preferably 0.1% by molebased on the content of M¹. When each of them contains Al and/or Sn asthe element M², the total content of Al and Sn is preferably 0.1% bymole or more, more preferably 0.15% by mole or more, particularlypreferably 0.2% by mole or more by mole or more based on the content ofM¹.

When the content of Ti, Zr, Ge, Nb, Al and/or Sn is too large in thelithium-containing transition metal oxide as the positive electrodeactive material (C), the load characteristic of the battery tends todecrease. Thus, when the oxide contains, as the element M², at least oneelement selected from Ti, Zr, Ge and Nb, the total content of Ti, Zr, Geand Nb is preferably less than 0.5% by mole, more preferably less than0.25% by mole, particularly preferably less than 0.15% by mole based onthe content of M¹. When the oxide contains Al and/or Sn as the elementM², the total amount of Al and/or Sn is preferably less than 1% by mole,more preferably less than 0.5% by mole, particularly preferably lessthan 0.3% by mole based on the content of M¹.

A method for including the metal element M² in the positive electrodeactive material (C) or the other lithium-containing transition metaloxides is not particularly limited. For example, the element M² may bepresent on the particles of the metal oxide, may be evenly present as asolid solution inside the metal oxides, or may be unevenly presentinside the metal oxides with having a density distribution. Furthermore,the element M² may form a compound which in turn forms a layer on theparticle surfaces. Preferably, the element M² is evenly present as asolid solution.

In the formulae (1) and (2) representing the positive electrode activematerial (C) and the other lithium-containing transition metal oxides,respectively, the element M³ is an element other than Li, M¹ and M². Thepositive electrode active material (C) and the other lithium-containingtransition metal oxides may each contain the M³ in an amount such thatthe advantageous effects of the present invention are not impaired, orthey may contain no M³.

Examples of the element M³ include alkali metals other than Li (e.g.,Na, K and Rb), alkaline earth metals other than Mg (e.g., Be, Ca, Sr andBa), Group IIIa metals (e.g., Sc, Y, La), Group IVa metals other than Tiand Zr (e.g., Hf), Group Va metals other than Nb (e.g., V and Ta), GroupVIa metals (e.g., Cr, Mo and W), Group VIIb metals other than Mn (e.g.,Tc and Re), Group VIII metals other than Co and Ni (e.g., Fe, Ru, andRh), Group Ib metals (e.g., Cu, Ag and Au), Group IIIb metals other thanZn and Al (e.g., B, Ga and In), Group IVb metals other than Sn and Pb(e.g., Si), P and Bi.

The metal element M² contributes to an improvement in the stability ofthe lithium-containing transition metal oxides. However, when thecontent thereof is too large, a function of storing and releasing Liions is impaired so that the battery characteristics may be decreased.Since the positive electrode active material (C) having the smallestaverage particle size has the particularly small particle size anddecreased stability, it is preferable that the content of the elementM², which is a stabilizing element, is somewhat high. In addition, sincethe positive electrode active material (C) has the small particle sizeand in turn the large surface area, it exhibits a high activity. Thus,the presence of the element M² in the material (C) has less influence onthe function of storing and releasing Li ions.

In contrast, the lithium-containing transition metal oxides havingrelatively large particle sizes, that is, the lithium-containingtransition metal oxides other than the positive electrode activematerial (C), have better stability than the positive electrode activematerial (C). Therefore, the former metal oxides have less necessity tocontain the element M² than the positive electrode active material (C).Furthermore, their function of storing and releasing Li ions is easilyimpaired by the presence of the element M² since the materials have thesmaller surface area and the lower activity than the positive electrodeactive material (C).

Accordingly, it is preferable that the content of the metal element M²in the positive electrode active material (C) is larger than that in thelithium-containing transition metal oxides other than the positiveelectrode active material (C).

When the three lithium-containing transition metal oxides having thedifferent average particle sizes are contained in the positiveelectrode, there is no especial limitation on the relationship of theelement M² content between the positive electrode active material (A)having the largest average particle size and the positive electrodeactive material (B) having the second largest average particle size.Thus, the former may contain a larger amount of the element M² than thelatter, and vise versa, or the element M² contents in the former and thelatter may be the same. Alternatively, neither the positive electrodeactive material (A) nor the positive electrode active material (B)contain the element M². In a more preferable embodiment, a metal oxidehaving a smaller average particle size contains a larger amount of theelement M². In particular, when the three lithium-containing transitionmetal oxides having different average particle sizes are used, theelement M² content in the positive electrode active material (C) havingthe smallest average particle size is largest, that in the positiveelectrode active material (B) having the second smallest averageparticle size is second largest, and that in the positive electrodeactive material (A) having the largest average particle size issmallest. In this case, the positive electrode active material (A) maycontain no element M².

In other words, when “c” in the formula (2) representing the positiveelectrode active material (B) is represented by “c¹”, and “c” in theformula (2) representing the positive electrode active material (A) isrepresented by “c²”, “z” in the formula (1), “c¹” and “c²” preferablysatisfy the following relationship: z>c¹>c² wherein c² may be zero.

When the positive electrode active materials (B) and (C) both containthe element M², z is preferably 1.1 times as large as c¹. When thepositive electrode active material (A) contains the element M², c¹ ismore preferably at least 2 times as large as c², particularly preferablyat least 2.5 times as large as c², particularly preferably at least 3times as large as c².

From the viewpoint of suppressing the decrease of load characteristic ofthe battery, z is preferably less than 4 times as large as c¹, morepreferably less than 3 times as large as c¹, particularly preferablyless than 2 times as large as c¹. When the positive electrode activematerial (A) contains M², c¹ is preferably less than 5 times as large asc² more preferably less than 4 times as large as c², particularlypreferably less than 3.5 times as large as c².

When the four or more lithium-containing transition metal oxides havingthe different average particle sizes are contained in the positiveelectrode, with regard to the lithium-containing transition metal oxidesother than the positive electrode active material (C) having thesmallest average particle size, no specific limitation is put on therelationship of the element M² content between the positive electrodeactive material (A) having the largest average particle size and thelithium-containing transition metal oxides other than the material (A).Thus, the element M² contents may be the same between a part or all ofthe lithium-containing transition metal oxides. The element M² contentsin the individual metal oxides may differ from each other. In a morepreferable embodiment, a metal oxide having a smaller average particlesize contains a larger amount of the element M².

In the lithium-containing transition metal oxides which constitute thepositive electrode active material according to the present invention,the oxides having different average particle sizes may have the samecomposition of elements, or different compositions of elements betweenthem. When the lithium-containing transition metal oxides according tothe present invention are the above-mentioned positive electrode activematerials (A), (B) and (C), the following combination may be used: acombination of the positive electrode active material (A) consisting ofLiCo_(0.9975)Mg_(0.001)Ti_(0.0005)Al_(0.001)O₂, the positive electrodeactive material (B) consisting ofLiCo_(0.3965)Ni_(0.30)Mn_(0.30)Mg_(0.0012)Ti_(0.0008)Al_(0.0015)O₂, andthe positive electrode active material (C) consisting ofLiCo_(0.1955)Ni_(0.8)Mg_(0.0015)Ti_(0.001)Al_(0.002)O₂.

The positive electrode active material comprising the lithium-containingtransition metal oxides according to the present invention is formedthrough a certain synthesizing process and a certain battery producingprocess. For example, for the preparation of lithium-containingtransition metal oxides which contain Co as the transition metal elementM¹ and have different average particle sizes, firstly, a solution of analkali such as NaOH is dropwise added to an acidic aqueous solutioncontaining Co to precipitate Co(OH)₂. In order to homogeneouslyprecipitate Co(OH)₂, Co may be coprecipitated with a different element,and then the coprecipitated material is calcined to obtain Co₃O₄. Theparticle size of the precipitates can be adjusted by controlling theperiod for forming the precipitates. The particle size of Co₃O₄ aftercalcination is also controlled by the particle size of the precipitateat this time.

When the positive electrode active material is synthesized, conditionssuch as a mixing condition, calcination temperature, calcinationatmosphere, calcination time, starting materials, and also batteryfabrication conditions are suitably selected. With regard to the mixingcondition in the synthesis of the positive electrode active material,preferably, for example, ethanol or water is added to the powderystarting materials, and then mixed in a planetary ball mill for 0.5 houror longer. More preferably, ethanol and water are mixed at a volumeratio of 50:50, and the mixture is agitated in a planetary ball mill for20 hours or longer. Through this mixing step, the powdery startingmaterials are sufficiently comminuted and mixed to prepare a homogeneousdispersion. The dispersion is dried with a spray drier or the like whilekeeping homogeneity. The calcination temperature is preferably from 750to 1,050° C., more preferably from 950 to 1,030° C. The calcinationatmosphere is preferably an air. The calcination time is preferably from10 to 60 hours, more preferably from 20 to 40 hours.

In the preparation of the positive electrode active material, Li₂CO₃ ispreferably used as a lithium source. As the sources of other metal suchas Mg, Ti, Ge, Zr, Nb, Al and Sn, preferred are nitrates or hydroxidesof these metals, or oxides thereof having a particle size of 1 μm orless. It is preferable to use the coprecipitate of the hydroxides sincethe different elements are uniformly distributed in the active material.

The positive electrode used in the present invention is formed by, forexample, a method described below. Firstly, the three or morelithium-containing transition metal oxides having different averageparticle sizes, for example, the positive electrode active materials(A), (B) and (C), are mixed with each other at a predetermined weightratio. If necessary, an electric conductive aid (e.g., graphite, carbonblack, acetylene black, etc.) is added to the mixture. Furthermore, tothe mixture, a binder (e.g., polyvinylidene fluoride, polytetrafluoroethylene, etc.) is added to prepare a positive electrodemixture. A solvent is used to formulate this positive electrode mixturein the form of a paste. The binder may be mixed with the positiveelectrode active material and the like after the binder is dissolved ina solvent. In this way, the paste containing the positive electrodemixture is prepared. The resultant paste is applied to a positiveelectrode current collector made of an aluminum foil or the like, andthen dried to form a positive electrode mixture layer. If necessary, thelayer is pressed to obtain a positive electrode. However, the method forproducing the positive electrode is not limited to the above-mentionedmethod, and may be any other method.

The positive electrode mixture layer according to the present inventionmay have a density of 3.8 g/cm³ or more, more preferably 3.85 g/cm³ ormore, particularly preferably 3.95 g/cm³ or more. With such a density,the capacity of the battery can be increased. However, when the densityof the positive electrode mixture layer is too high, the wettabilitywith the nonaqueous electrolyte, which will be explained later,decreases. Thus, the density is preferably 4.6 g/cm³ or less, morepreferably 4.4 g/cm³ or less, particularly preferably 4.2 g/cm³ or less.

Herein, the density of the positive electrode mixture layer may beobtained by the following measuring method: The positive electrode iscut to form a sample piece having a predetermined area, the sample pieceis weighed with an electronic balance having a minimum scale of 1 mg,and then the weight of the current collector is subtracted from theweight of the sample piece to calculate the weight of the positiveelectrode mixture layer. The total thickness of the positive electrodeis measured at ten points with a micrometer having a minimum scale of 1μm. Then, the thickness of the current collector is subtracted from theresultant individual thicknesses, and the thicknesses of the positiveelectrode mixture layer measured at ten points are averaged. From theaveraged thicknesses of the positive electrode mixture layer and thesurface area, the volume of the positive electrode mixture layer iscalculated. Finally, the weight of the positive electrode mixture layeris divided by the volume thereof to obtain the density of the positiveelectrode mixture layer.

The thickness of the positive electrode mixture layer is preferably from30 to 200 μm, and the thickness of the current collector used in thepositive electrode is preferably from 8 to 20 μm.

In the positive electrode mixture layer, the content of thelithium-containing transition metal oxides as the active materials ispreferably 96% by weight or more, more preferably 97% by weight or more,particularly preferably 97.5% by weight or more, while it is preferably99% by weight or less, more preferably 98% by weight or less. Thecontent of the binder in the positive electrode mixture layer ispreferably 1% by weight or more, more preferably 1.3% by weight or more,particularly preferably 1.5% by weight or more, while it is preferably4% by weight or less, more preferably 3% by weight or less, particularlypreferably 2% by weight or less. The content of the electric conductiveaid in the positive electrode mixture layer is preferably 1% by weightor more, more preferably 1.1% by weight or more, particularly preferably1.2% by weight or more, while it is preferably 3% by weight or less,more preferably 2% by weight or less, particularly preferably 1.5% byweight or less.

When the content of the active material in the positive electrodemixture layer is too small, the capacity cannot be increased and alsothe density of the positive electrode mixture layer cannot be increased.When this content is too large, the resistance may increase or theformability of the positive electrode may be impaired. When the bindercontent in the positive electrode mixture layer is too large, thecapacity may hardly be increased. When this content is too small, theadhesion of the layer to the current collector decreases so that thepowder may drop off from the electrode. Thus, the above-mentionedpreferable ranges are desirable. Furthermore, when the content of theelectric conductive aid in the positive electrode mixture layer is toolarge, the density of the positive electrode mixture layer may not bemade sufficiently high so that the capacity may hardly be increased.When this content is too small, the sufficient electric conductionthrough the positive electrode mixture layer is not attained so that thecharge-discharge cycle characteristic or the load characteristic of thebattery may deteriorated.

It is essential for the nonaqueous secondary battery of the presentinvention to have the positive electrode explained above, and thus thereis no specific limitation on other elements or structure of the battery.The battery of the present invention may adopt various elements andstructures, which are commonly adopted in the conventional nonaqueoussecondary batteries in the state of art.

The negative electrode active material in the negative electrode may beany material that can be doped and de-doped with Li ions. Examplesthereof are carbonaceous materials such as graphite, pyrolytic carbons,cokes, glassy carbons, burned bodies of organic polymers, mesocarbonmicrobeads, carbon fibers and activated carbon. In addition, thefollowing materials can also be used as the negative electrode activematerial: alloys of Si, Sn, In or the like, oxides of Si, Sn or the likethat can be charged and discharged at a low voltage near a voltage atwhich Li can be charged and discharged, and nitrides of Li and Co suchas Li_(2.6)Co_(0.4)N. Graphite can be partially substituted with ametal, a metal oxide or the like that can be alloyed with Li. Whengraphite is used as the negative electrode active material, the voltagewhen the battery is fully charged can be regarded as about 0.1 V withreference to the potential of lithium, and therefore the voltage of thepositive electrode can be conveniently calculated as a voltage obtainedby adding 0.1 V to the battery voltage. Consequently, the charge voltageof the positive electrode is easily controlled.

Preferably, graphite has such a form in that a lattice spacing d₀₀₂ ofthe (002) planes is 0.338 nm or less, since the negative electrode or anegative electrode mixture layer, which will be explained later, has ahigher density as the crystallinity is higher. However, when the latticespacing d₀₀₂ is too large, the high density negative electrode maydecrease the discharge characteristic or the load characteristic of thebattery. Thus, the lattice spacing d₀₀₂ is preferably 0.335 nm or more,more preferably 0.3355 nm or more.

The crystal size of the graphite in the c axis direction (Lc) ispreferably 70 nm or more, more preferably 80 nm or more, particularlypreferably 90 nm or more. As the Lc is larger, the charging curvebecomes flat so that the voltage of the positive electrode is easilycontrolled and also the capacity can be made large. When the Lc is toolarge, the battery capacity tends to decrease with the high-densitynegative electrode. Thus, the Lc is preferably less than 200 nm.

Furthermore, the specific surface area of the graphite is preferably 0.5m²/g or more, more preferably 1 m²/g or more, particularly preferably 2m²/g or more, while it is preferably 6 m²/g or less, more preferably 5m²/g or less. Unless the specific surface area of the graphite issomewhat large, the characteristics tend to decrease. When the specificsurface area is too large, the graphite easily reacts with theelectrolyte and such a reaction may have influences on the properties ofthe battery.

The graphite used in the negative electrode is preferably made ofnatural graphite. More preferred is a mixture of two or more graphitematerials having different surface crystallinity to achieve the highdensity of the negative electrode. Since natural graphite is inexpensiveand achieves a high capacity, the negative electrode with a high costperformance can be produced. Usually, when natural graphite is used, thebattery capacity is easily decreased as the density of the negativeelectrode is increased. However, the decrease in the battery capacitycan be suppressed by mixing the natural graphite with a graphite havinga reduced surface crystallinity by a surface treatment.

The surface crystallinity of specific graphite can be determined by theRaman spectrum analysis. When the R value of the Raman spectrum(R=I₁₃₅₀/I₁₅₈₀, that is, the ratio of the Raman intensity around 1350cm⁻¹ to that around 1580 cm⁻¹) is 0.01 or more, where the Raman spectrumis measured with graphite which has been excited with an argon laserhaving a wavelength of 514.5 nm, the surface crystallinity of thespecific graphite is slightly lower than that of natural graphite. Thus,with the graphite having a surface crystallinity decreased by thesurface treatment, the R value is preferably 0.01 or more, morepreferably 0.1 or more, while it is preferably 0.5 or less, morepreferably 0.3 or less. The content of the graphite having a surfacecrystallinity decreased by surface treatment is preferably 100% byweight of the whole graphite in order to increase the density of thenegative electrode. However, in order to prevent the decrease of thebattery capacity, the content of such graphite is preferably 50% byweight or more, more preferably 70% by weight or more, particularlypreferably 85% by weight or more of the whole graphite.

When the average particle size of the graphite is too small, anirreversible capacity increases. Thus, the average particle size of thegraphite is preferably 5 μm or more, more preferably 12 μm or more,particularly preferably 18 μm or more. From the viewpoint of theincrease of the capacity of the negative electrode, the average particlesize of the graphite is 30 μm or less, more preferably 25 μm or less,particularly preferably 20 μm or less.

The negative electrode may be produced by the following method, forexample: The negative electrode active material and an optional a binderand/or other additives are mixed to prepare a negative electrodemixture, and the mixture is dispersed in a solvent to prepare a paste.Preferably, the binder is dissolved in a solvent prior to mixing withthe negative electrode active material, and then mixed with the negativeelectrode active material and so on. The paste containing the negativeelectrode mixture is applied to a negative electrode current collectormade of a copper foil or the like, and then dried to form a negativeelectrode mixture layer. The layer is pressed to obtain a negativeelectrode. However, the method for producing the negative electrode isnot limited to the above-mentioned method, and may be any other method.

The density of the negative electrode mixture layer after pressing ispreferably 1.70 g/cm³ or more, more preferably 1.75 g/cm³ or more. Basedon the theoretical density of graphite, the upper limit of the densityof the negative electrode mixture layer formed using graphite is 2.1 to2.2 g/cm³. The density of the negative electrode mixture layer ispreferably 2.0 g/cm³ or less, more preferably 1.9 g/cm³ or less from theviewpoint of the affinity with the nonaqueous electrolyte. It ispreferable to press the negative electrode plural times since thenegative electrode can be uniformly pressed.

The binder used in the negative electrode is not particularly limited.For the increase of the content of the active material to increase thecapacity, the amount of the binder is preferably made as small aspossible. To this end, the binder is preferably a mixture of an aqueousresin which can be dissolved or dispersed, and a rubbery polymer, sincethe use of only a small amount of the aqueous resin can contribute tothe dispersion of the graphite and thus prevents the delamination of thenegative electrode mixture layer from the current collector caused bythe expansion and contraction of the electrode in the charge-dischargecycles.

Examples of the aqueous resins include cellulose resins such ascarboxymethylcellulose, and hydroxypropylcellulose, andpolyvinylpyrrolidone, polyepichlorohydrin, polyvinylpyridine, polyvinylalcohol, polyether resins such as polyethylene oxide and polyethyleneglycol, etc. Examples of the rubbery polymers include latex, butylrubber, fluororubber, styrene-butadiene rubber, nitrile-butadienecopolymer rubber, ethylene-propylene-diene copolymer (EPDM),polybutadiene, etc. From the viewpoint of the dispersibility of thegraphite particles and the prevention of delamination of the layer, itis preferable to use a cellulose ether compound such ascarboxymethylcellulose together with a butadiene copolymer rubber suchas a styrene-butadiene rubber. It is particularly preferable to usecarboxymethylcellulose together with a butadiene copolymer rubber suchas a styrene-butadiene copolymer rubber or a nitrile-butadiene-copolymerrubber. The cellulose ether compound such as carboxymethylcellulosemainly has a thickening effect on the paste containing the negativeelectrode mixture, while the rubbery polymer such as thestyrene-butadiene copolymer rubber has a binding effect on the negativeelectrode mixture. When the cellulose ether compound such ascarboxymethylcellulose and the rubbery polymer such as thestyrene-butadiene copolymer rubber are used in combination, the weightof the former to the latter is preferably from 1:1 to 1:15.

The thickness of the negative electrode mixture layer is preferably from40 to 200 μm. The thickness of the current collector used in thenegative electrode is preferably from 5 to 30 μm.

In the negative electrode mixture layer, the content of the binder orbinders is preferably 1.5% by weight or more, more preferably 1.8% byweight or more, particularly preferably 2.0% by weight or more of thelayer, while it is preferably less than 5% by weight, less than 3% byweight, less than 2.5% by weight. When the amount of the binder in thenegative electrode mixture layer is too large, the discharge capacity ofthe battery may decrease. When the amount is too small, the adhesionbetween the particles decreases. The content of the negative electrodeactive material in the negative electrode mixture layer is preferablymore than 95% by weight and 98.5% by weight or less.

In the nonaqueous secondary battery of the present invention, thenonaqueous electrolyte is preferably a nonaqueous solvent-baseelectrolytic solution comprising an electrolyte salt such as a lithiumsalt dissolved in a nonaqueous solvent such as an organic solvent, fromthe viewpoint of electric characteristics or handling easiness. Apolymer electrolyte or a gel electrolyte may be used without anyproblem.

The solvent in the nonaqueous electrolytic solution is not particularlylimited, and examples thereof include acyclic esters such as dimethylcarbonate, diethyl carbonate, ethylmethyl carbonate, and methyl propylcarbonate; cyclic esters having a high dielectric constant, such asethylene carbonate, propylene carbonate, butylene carbonate, andvinylene carbonate; and mixed solvents comprising an acyclic ester and acyclic ester. Mixed solvents each comprising an acyclic ester as a mainsolvent and a cyclic ester are particularly suitable.

Apart form the esters exemplified above, the following solvents may alsobe used: acyclic phosphoric acid triesters such as trimethyl phosphate;ethers such as 1,2-dimethoxyethane, 1,3-dioxolane, tetrahydrofuran,2-methyl-tetrahydrofuran and diethyl ether; nitriles and dinitriles;isocyanates; and halogen-containing solvents. Furthermore, amine orimide organic solvents, or sulfur-containing organic solvents such assulfolane may be used.

Furthermore, the nonaqueous electrolytic solution may preferably containa nonionic aromatic compound. Specific examples thereof include aromaticcompounds having an alkyl group bonded to an aromatic ring (e.g.,cyclohexylbenzene, isopropylbenzene, tert-butylbenzene,tert-amylbenzene, octylbenzene, toluene and xylene); aromatic compoundshaving a halogen group bonded to an aromatic ring (e.g., fluorobenzene,difluorobenzene, trifluorobenzene and chlorobenzene); aromatic compoundshaving an alkoxy group bonded to an aromatic ring (e.g., anisole,fluoroanisole, dimethoxybenzene and diethoxybenzene); aromaticcarboxylic acid esters such as phthalic acid esters (e.g., dibutylphthalate and di-2-ethylhexyl phthalate) and benzoic acid esters;carbonic acid esters having a phenyl group (e.g., methylphenylcarbonate, butylphenyl carbonate and diphenyl carbonate); phenylpropionate; and biphenyl. Among them, the compounds having an alkylgroup bonded to an aromatic ring (alkaryl compounds) are preferred, andcyclohexylbenzene is particularly preferred.

The aromatic compounds exemplified above can form a film on the surfaceof the active material in the positive electrode or the negativeelectrode in the battery. These aromatic compounds may be used alone,while more advantageous effects can be attained by the use of two ormore of the aromatic compounds together. Particularly advantageouseffects can be attained on the improvement of the safety of the batteryby the use of the alkaryl compound together with an aromatic compound,which can be oxidized at a lower voltage than the alkaryl compound, suchas biphenyl.

The method for adding the aromatic compound in the nonaqueouselectrolytic solution is not particularly limited. In general, thearomatic compound is added to the nonaqueous electrolytic solution priorto the fabrication of the battery.

The content of the aromatic compound in the nonaqueous electrolyticsolution is preferably 4% by weight or more from the viewpoint of thesafety, and it is preferably 10% by weight or less from the viewpoint ofthe load characteristic. When two or more aromatic compounds are usedtogether, the total amount thereof is within the above-mentioned range.When the alkaryl (alkylaryl) compound and the aromatic compound whichcan be oxidized at a lower voltage that the alkaryl compound are used incombination, the content of the alkaryl compound in the nonaqueouselectrolytic solution is preferably 0.5% by weight or more, morepreferably 2% by weight or more, while it is preferably 8% by weight orless, more preferably 5% by weight or less. The content of the aromaticcompound that can be oxidized at a lower voltage than the alkarylcompound in the nonaqueous electrolytic solution is preferably 0.1% byweight or more, more preferably 0.2% by weight or more, while it ispreferably 1% by weight or less, more preferably 0.5% by weight or less.

Furthermore, a surface protecting coating can be formed on the surfaceof the positive electrode active material in the step of initialcharging of the battery, when the nonaqueous electrolytic solutioncontains at least one compound selected from the group consisting ofhalogen-containing organic solvents (e.g., halogen-containingcarbonates), sulfur-containing organic solvents, fluorine-containingorganic lithium salts, phosphorus-containing organic solvents,silicon-containing organic solvents, nitrogen-containing organicsolvents, etc. Among them, organic fluorine-containing solvents (e.g.,fluorine-containing carbonates), organic sulfur-containing solvents andfluorine-containing organic lithium salts are more preferable. Specificexamples thereof include F-DPC [C₂F₅CH₂O(C═O)OCH₂C₂F₅], F-DEC[CF₃CH₂O(C═O)OCH₂CF₃], HFE7100 (C₄F₉OCH₃), butyl sulfate(C₄H₉OSO₂OC₄H₉), methylethylene sulfate [(—OCH(CH₃)CH₂O—)SO₂], butylsulfate (C₄H₉SO₂C₄H₉), a polymer imide salt([—N(Li)SO₂OCH₂(CF₂)₄CH₂OSO₂—]_(n) wherein n is from 2 to 100),(C₂F₅SO₂)₂NLi, and [(CF₃)₂CHOSO₂]₂NLi.

Such additives may be used alone. It is particularly preferable to use afluorine-containing organic solvent together with a fluorine-containingorganic lithium salt. The amount of such an additive added is preferably0.1% by weight or more, more preferably 2% by weight or more,particularly preferably 5% by weight or more, while it is preferably 30%by weight or less, more preferably 10% by weight or less, each based onthe whole weight of the nonaqueous electrolytic solution. When theamount of the additive is too large, the electric characteristics of thebattery may deteriorate. When the amount is too small, a good coatingmay hardly be formed.

When the battery comprising the nonaqueous electrolytic solutioncontaining the above-mentioned additive(s) is charged, particularly at ahigh voltage, a surface protecting coating that contains fluorine orsulfur atoms is formed on the positive electrode active materialsurface. This surface protecting coating may contain either fluorineatoms or sulfur atoms. Preferably, the coating contains both of fluorineatoms and sulfur atoms.

The amount of the sulfur atoms in the surface protecting coating formedon the positive electrode active material surface is preferably 0.5atomic % or more, more preferably 1 atomic % or more, particularlypreferably 3 atomic % or more. However, when the amount of the sulfuratoms in the positive electrode active material surface is too large,the discharge characteristic of the battery tends to decrease. Thus, theamount is preferably 20 atomic % or less, more preferably 10 atomic % orless, particularly preferably 6 atomic % or less.

The amount of the fluorine atoms in the surface protecting coatingformed on the positive electrode active material surface is preferably15 atomic % or more, more preferably 20 atomic % or more, particularlypreferably 25 atomic % or more. However, when the amount of the fluorineatoms in the positive electrode active material surface is too large,the discharge characteristic of the battery tends to fall. Thus, theamount of the fluorine atoms is preferably 50 atomic % or less, morepreferably 40 atomic % or less, particularly preferably 30 atomic % orless. The surface protecting coating, which contains the fluorine atomsand/or the sulfur atoms, in the positive electrode active material maynot be formed by charging the battery as described above, but thepositive electrode (battery) may be formed by the use of the positiveelectrode active material, that is, the lithium-containing transitionmetal oxides, which has such a surface protecting coating alreadyformed.

In order to improve the charge-discharge cycle characteristic of thebattery, preferably, the nonaqueous electrolytic solution contains atleast one carbonate compound selected from the group consisting ofvinylene carbonates, such as (—OCH═CHO—)C═O, (—OCH═C(CH₃)O—)C═O and(—OC(CH₃)═C(CH₃)O—)C═O, and derivatives thereof; andfluorine-substituted ethylene carbonates, such as (—OCH₂—CHFO—)C═O and(—OCHF—CHFO—)C═O. The addition amount thereof is preferably 0.1% byweight or more, more preferably 0.5% by weight or more, particularlypreferably 2% by weight or more based on the whole weight of thenonaqueous electrolytic solution. When the addition amount thereof istoo large, the load characteristic of the battery tends to decrease.Thus, the addition amount is preferably 10% by weight or less, morepreferably 5% by weight or less, particularly preferably 3% by weight orless based on the whole weight of the nonaqueous electrolytic solution.

Examples of the electrolyte salt to be dissolved in the solvent duringthe preparation of the nonaqueous electrolytic solution include LiClO₄,LiPF₆, LiBF₄, LiAsF₆, LiSbF₆, LiCF₃SO₃, LiC₄F₉SO₃, LiCF₃CO₂,Li₂C₂F₄(SO₃)₂, LiN(RfSO₂) (Rf′SO₂), LiC(RfSO₂)₃, LiC_(n)F_(2n+1)SO₃wherein n≧2, and LiN(RfOSO₂)₂ wherein Rf and Rf′ each represent afluoroalkyl group. They may be used alone or in combination of two ormore thereof. Among these electrolyte salts, particularly preferred arefluorine-containing organic lithium salts having 2 or more carbon atoms,since such lithium salts have a large anionic property and further ionseparation easily occurs so that the salts are easily dissolved in theabove-mentioned solvents. The concentration of the electrolyte salt inthe nonaqueous electrolytic solution is not particularly limited, and itis preferably 0.3 mol/L or more, more preferably 0.4 mol/L or more,while it is preferably 1.7 mol/L or less, more preferably 1.5 mol/L orless.

In the present invention, the nonaqueous electrolyte may be a gel-formpolymer electrolyte besides the nonaqueous electrolytic solutiondescribed above. The gel-form polymer electrolyte corresponds to aproduct obtained by the gelation of the nonaqueous electrolytic solutionwith a gelling agent. For the gelation of the nonaqueous electrolyticsolution, the following gelling agents may be used: a linear polymersuch as polyvinylidene fluoride, polyethylene oxide orpolyacrylonitrile, or a copolymer thereof; or a polyfunctional monomerwhich can be polymerized by irradiation with actinic rays such asultraviolet rays or electron beams (e.g., an acrylate having 4 or morefunctionalities such as pentaerythritol tetraacrylate,ditrimethylolpropane tetraacrylate, ethoxylated pentaerythritoltetraacrylate, dipentaerythritol hydroxypentaacrylate, ordipentaerythritol hexaacrylate; or a methacrylate having 4 or morefunctionalities, which are analogous to the above acrylates. In the caseof the monomer, the monomer itself does not cause the gelling of theelectrolytic solution, but the polymer formed from the monomer acts as agelling agent.

When a polyfunctional monomer is used to gel the electrolytic solutionas described above, a polymerization initiator may optionally be used.Examples of the polymerization initiator include benzoyls, benzoin alkylethers, benzophenones, benzoylphenylphosphine oxides, acetophenones,thioxanthones and anthraquinones. As a sensitizer for the polymerizationinitiator, an alkylamine or an aminoester may be used.

In the present invention, the nonaqueous electrolyte may be a solidelectrolyte besides the nonaqueous electrolytic solution or the gel-formpolymer electrolyte. The solid electrolyte may be an inorganic solidelectrolyte or an organic solid electrolyte.

In the present invention, a separator used in the present inventionpreferably has a thickness of 5 μm or more, more preferably 10 μm ormore, particularly preferably 12 μm or more, while it is preferably lessthan 25 μm, more preferably less than 20 μm, particularly preferablyless than 18 μm, from the viewpoint of imparting the directionality ofthe tensile strength to the separator, keeping good insulatingproperties and reducing the thermal shrinkage of the separator. The gaspermeability of the separator is preferably 500 second/100-mL or less,more preferably 300 second/100-mL or less, particularly preferably 120second/100-mL or less. As the gas permeability of the separator issmaller, the load characteristic is made better but an insideshort-circuit is more easily caused. Thus, the gas permeability ispreferably 50 second/100-mL or more. Here, a gas permeability ismeasured according to JIS P8117. As the thermal shrinkage factor of theseparator in the transverse direction (TD) is smaller, an insideshort-circuit is less easily caused when the temperature of the batteryrises. Thus, the thermal shrinkage factor in TD of the separator is assmall as possible. The thermal shrinkage factor in TD is preferably 10%or less, more preferably 5% or less. In order to restrain the thermalshrinkage of the separator, it is preferable to thermally treat theseparator in advance at a temperature of about 100 to 125° C. Theseparator having such a thermal shrinkage factor is preferably combinedwith the positive electrode materials according to the present inventionto fabricate a battery, since the behaviors of the battery at hightemperature become stable.

The thermal shrinkage factor in TD of the separator means the shrinkagefactor of a portion thereof that most largely shrinks in TD when theseparator having a size of 30 mm square is allowed to stand at 105° C.for 8 hours.

With regard to the strength of the separator, a tensile strength in themachine direction (MD) is preferably 6.8×10⁷ N/m² or more, morepreferably 9.8×10⁷ N/m or more. The tensile strength in TD is preferablysmaller than that in MD. The ratio of the tensile strength in TD to thatin MD (tensile strength in TD/tensile strength in MD) is preferably 0.95or less, more preferably 0.9 or less, while it is preferably 0.1 ormore. The transverse direction means a direction perpendicular to thedirection in which the film resin for the production of the separator iswound up, that is, the machine direction.

The puncture strength of the separator is preferably 2.0 N or more, morepreferably 2.5 N or more. As this value is higher, the battery is lesseasily short-circuited. Usually, however, the upper limit thereof issubstantially determined by the material of the separator. In the caseof a separator made of polyethylene, the upper limit of the puncturestrength is about 10 N. Here, a puncture strength is measured by cuttinga sample piece of 50 mm×50 mm from a separator, clamping the samplepiece with jigs at the edges of 5 mm, puncturing the sample piece with aneedle having a tip end with a radius of 0.5 mm at a rate of 2 mm/sec.,and measuring a maximum load before the puncture of the sample piece asa puncture strength.

When a conventional nonaqueous secondary battery is charged at a highpositive electrode voltage of 4.35 V or higher with reference to thepotential of lithium and is discharged to a final voltage higher than3.2 V, the crystalline structure of the positive electrode activematerial decays to decrease the capacity or to induce heating of thebattery due to the deterioration of the thermal stability. Thus, thebattery may not be practically used. When a positive electrode activematerial to which a different element such as Mg or Ti is added is used,the decrease of the safety or of the capacity over charge-dischargecycles can be suppressed, but the degree of suppression is notsufficient. Moreover, the filling of the positive electrode isinsufficient so that the battery easily expands.

In contrast, the battery of the present invention having the structureexplained above is a nonaqueous secondary battery which improves thecapacity, the charge-discharge cycle characteristic, the safety and thesuppression of expansion of the battery. These advantageous effects canbe attained at a usual charging volt (a battery voltage of 4.2 V).Furthermore, when the positive electrode is charged up to a high voltageof 4.35 V with reference to the potential of lithium (i.e., a batteryvoltage of 4.25 V) and then the discharge of the battery is terminatedat a high voltage, that is, a battery voltage of 3.2 V or higher, thecrystalline structures of the positive electrode active materials arevery stable so that the decrease of the capacity or thermal stability isprevented.

Moreover, the positive electrode active material of any conventionalnonaqueous secondary battery generates a low average voltage. Therefore,when a charge-discharge cycle test is repeated under a condition thatthe discharge final voltage of a unit cell is 4.35 V or higher withreference to the potential of lithium, the positive electrode is dopedor dedoped with a large amount of Li ions. This situation is analogousto a case where the battery is subjected to a charge-discharge cycletest under an overcharge condition. Under such a severe condition, anyconventional positive electrode active material cannot maintain itscrystalline structure so as to cause disadvantages such that the thermalstability declines or the charge-discharge cycle life is shortened. Tothe contrary, the use of the positive electrode active materialsaccording to the battery of the present invention can overcome suchdisadvantages of the conventional positive electrode active material.Thus, the present invention provides a nonaqueous secondary batterywhich can be reversibly charged and discharged even at a high voltage,such as a voltage of 4.35 to 4.6 V with reference to the potential oflithium.

The nonaqueous secondary battery of the present invention hascharacteristics including a high voltage, a high capacity and a highsafety. By making use of such characteristics, the nonaqueous secondarybattery of the present invention can be used as a power source of anotebook personal computer, a stylus-operated personal computer, apocket personal computer, a notebook word processor, a pocket wordprocessor, an electronic book player, a cellular phone, a codelesshandset, a pager, a portable terminal, a portable copier, an electricalnotebook, an electronic calculator, a liquid crystal television set, anelectric shaver, an electric power tool, an electronic translatingmachine, an automobile telephone, a transceiver, a voice input device, amemory card, a backup power source, a tape recorder, a radio, aheadphone stereo, a handy printer, a handy cleaner, a portable CDplayer, a video movie, a navigation system, a refrigerator, an airconditioner, a television, a stereo, a water heater, a microwave oven, adishwasher, a washing machine, a drying machine, a game equipment, alighting equipment, a toy, a sensor equipment, a load conditioner, amedical machine, an automobile, an electric vehicle, a golf cart, anelectrically-powered cart, a security system, a power storing system, orthe like. The battery can be used not only for the consumer applicationsbut also for aerospace applications. The capacity-increasing effect ofthe present invention is enhanced, in particular, in small-sizedportable devices. Thus, the battery of the present invention is used ina portable device desirably having a weight of 3 kg or less, moredesirably 1 kg or less. The lower limit of the weight of the portabledevice is not particularly limited. However, the lower limit isdesirably a value equal to the weight of the battery, for example, 10 gor more in order to attain the advantageous effects to some degree.

EXAMPLES

The present invention will be described in detail with reference to thefollowing Examples; however, the Examples do not limit the scope of thepresent invention. Thus, modifications of the examples are encompassedby the scope of the present invention as long as the modifications donot depart from the subject matter of the present invention, which hasbeen described above or will be described hereinafter.

Example 1 Production of Positive Electrode

The lithium-containing positive electrode materials,LiCo_(0.9978)Mg_(0.0008)Ti_(0.0004)Al_(0.001)O₂ (average particle size:18 μm) as a positive electrode active material (A),LiCo_(0.9949)Mg_(0.0024)Ti_(0.0012)Al_(0.0015)O₂ (average particle size:5 μm) as a positive electrode active material (B), andLiCo_(0.9940)Mg_(0.0027)Ti_(0.0013)Al_(0.0020)O₂ (average particle size:1 μm) as a positive electrode active material (C) at a weight ratio of77:19:4 were mixed. Then, 97.3 parts by weight of the mixture and 1.5parts by weight of a carbonaceous material as an electric conductive aidwere charged in a volumetric feeder as a device for supplying powder. Anamount of a 12 wt. % solution of polyvinylidene fluoride (PVDF) inN-methyl-2-pyrrolidone (NMP) to be supplied to the feeder was adjustedto control a solid content in the mixture constantly at 94% by weightduring kneading. While the amount of the mixed materials supplied in aunit time was controlled to a predetermined amount, the materials weresupplied in a biaxial kneading extruder and then kneaded. In this way, apaste containing the positive electrode mixture was prepared. Theresultant paste was charged in a planetary mixer, and then a 12 wt. %solution of PVDF in NMP, and NMP were added to dilute the paste, therebyadjusting the viscosity of the paste at a level sufficient forapplication. This diluted paste containing the positive electrode activematerial mixture was passed through a 70-mesh net to remove largesubstances. Thereafter, the paste was uniformly applied to both surfacesof a positive electrode current collector made of an aluminum foil witha thickness of 15 μm, and then dried to form film-form positiveelectrode mixture layers. In the dried positive electrode mixturelayers, the weight ratio of the positive electrode active material/theelectric conduction aiding agent/PVDF was 97:1.5:1.5. Thereafter, theresultant sheet was pressed and cut out in a predetermined size. To thecut piece, a lead member made of aluminum was welded to form asheet-form positive electrode. The density of the pressed positiveelectrode mixture layers (the density of the positive electrode) was3.95 g/cm³. The thickness of the positive electrode mixture layers (thetotal thickness of the layers on both the surfaces, i.e., the thicknessobtained by subtracting the thickness of the aluminum foil layer of thepositive electrode current collector from the total thickness of thepositive electrode) was 130 μm.

In the positive electrode active material (A), the amount of Mg was0.08% by mole, that of Ti was 0.04% by mole, and that of Al was 0.1% bymole, each based on the amount of Co. An electron prove X-raymicroanalyzer (EMPA 1600 manufactured by Shimadzu Corporation) was usedto measure the concentration of the metal element M² in cross sectionsof the particles. As a result, no difference in the concentration ofeach of Mg, Ti and Al was observed between the surface portion and thecore portion.

In the positive electrode active material (B), the amount of Mg was0.24% by mole, that of Ti was 0.12% by mole, and that of Al was 0.15% bymole, each based on the amount of Co. The concentration of the metalelement M² in the cross sections of the particles was measured in thesame manner as in the case of the positive electrode active material(A). As a result, no difference in the concentration of each of Mg, Tiand Al was observed between the surface portion and the core portion.

In the positive electrode active material (C), the amount of Mg was0.27% by mole, that of Ti was 0.13% by mole, and that of Al was 0.2% bymole, each based on the amount of Co. The concentration of the metalelement M² in cross sections of the particles was measured in the samemanner as in the case of the positive electrode active material (A). Asa result, no difference in the concentration of each of Mg, Ti and Alwas observed between the surface portion and the core portion.

The above-mentioned results show that, among the positive electrodeactive materials used in Example 1, the content of the metal element M²was larger as the average particle size was smaller, and the positiveelectrode active material (C) having the smallest average particle sizecontained the largest amount of the metal element M².

Production of Negative Electrode

In a planetary mixer, 180 parts by weight of a graphite typecarbonaceous material (purity: 99.9% or more, average particle size: 18μm, d₀₀₂: 0.337 nm, size of the crystallite in the c axis direction(Lc): 95.0 nm) as a negative electrode active material and 65 parts byweight of a 12 wt. % solution of PVDF in NMP were charged, and then themixture was kneaded for 1 hour to prepare a paste containing negativeelectrode mixture. The resultant paste had a solid content of 60.0% byweight. Thereafter, a 12 wt. % solution of PVDF in NMP, and NMP wereadded to the above mixture to adjust the viscosity of the paste at alevel sufficient for application. This paste containing negativeelectrode mixture was passed through a 70-mesh net to remove largeinclusions. Thereafter, the paste was uniformly applied to both surfacesof a negative electrode current collector made of a strip-form copperfoil having a thickness of 10 μm, and then dried to form negativeelectrode mixture layers. The resultant sheet was pressed until thedensity of the negative electrode mixture layers became 1.55 g/cm³. Theresultant sheet was then cut out in a predetermined size. Thereafter, alead member made of nickel was welded to the cut piece to form asheet-form negative electrode.

Preparation of Nonaqueous Electrolytic Solution

An amount of LiPF₆ was dissolved in a mixed solvent of methylethylcarbonate, diethyl carbonate and ethylene carbonate mixed at a volumeratio of 1.5:0.5:1 to attain a concentration of 1.2 mol/L. To thissolution, 3% by weight of fluorobenzene, 0.2% by weight of biphenyl,0.5% by weight of propanesultone, 10% by weight of C₄F₉OCH₃, and 3% byweight of vinylene carbonate were added to prepare a nonaqueouselectrolytic solution.

Production of Nonaqueous Secondary Battery

The positive electrode and the negative electrode were dried, and thenspirally wound with interposing, therebetween, a separator made of amicroporous polyethylene film (porosity: 53%, tensile strength in MD:2.1×10⁸ N/m², tensile strength in TD: 0.28×10⁸ N/m², thickness: 16 μm,gas permeability: 80 seconds/100-mL, thermal shrinkage factor afterbeing kept at 105° C. for 8 hours: 3%, puncture strength: 3.5 N (360g)), to form an electrode body having a spiral structure. Thereafter,the electrode body was pressed to form a flat-shaped electrode body andinserted into a box-shaped battery case made of an aluminum alloy. Thepositive and negative lead members were welded and a cover plate waslaser welded to the edge portion of an opening of the battery case.Then, the nonaqueous electrolytic solution prepared in the above waspoured into the battery case through an inlet made in the cover plate.The nonaqueous electrolytic solution was sufficiently infiltrated intothe separator and the like. Thereafter, the battery was partiallycharged, and gas generated during the partial charging was discharged.Then, the inlet was sealed up to make the battery airtight. Thereafter,the battery was charged and aged to yield a rectangular nonaqueoussecondary battery having a structure as shown in FIGS. 1A and 1B and anexternal appearance as shown in FIG. 2, and a width of 34.0 mm, athickness of 4.0 mm, and a height of 50.0 mm.

Here, the battery shown in FIGS. 1A, 1B and 2 will be explained. Thepositive electrode 1 and the negative electrode 2 are spirally woundwith interposing the separator 3 therebetween, as described above, andthe spirally wound electrode body is pressed in a flat form to form theelectrode laminate 6 having a flat spiral structure. The laminate 6together with a nonaqueous electrolytic solution is contained in thebox-shaped battery case 4. For simplicity, in FIG. 1, metal foils ascurrent collectors used to form the positive electrode 1 and thenegative electrode 2, and the electrolytic solution are not depicted.

The battery case 4 is made of an aluminum alloy, and constitutes a mainpart of the exterior package of the battery. This battery case 4 alsofunctions as a positive electrode terminal. The insulator 5 made of apolytetrafluoroethylene sheet is arranged on the inside bottom of thebattery case 4. The positive electrode lead member 7 and the negativeelectrode lead member 8 connected to one end of the positive electrode 1and that of the negative electrode 2, respectively, are taken out fromthe electrode laminate 6 having the flat spiral structure. The terminal11 made of stainless steel is attached to the cover plate 9 made ofaluminum for closing the opening of the battery case 4 with interposingthe insulation packing 10 made of polypropylene therebetween. The leadplate 13 made of stainless steel is attached to this terminal 11 withinterposing the insulator 12 therebetween.

The cover plate 9 is inserted into the opening of the battery case 4,and their joining portions are welded to each other, thereby closing theopening of the battery case 4 to make the interior of the batteryairtight. In the battery shown in FIGS. 1A and 1B, the inlet 14 forpouring the electrolytic solution is made in the cover plate 9, and theinlet 14 is welded and sealed up by, for example, laser welding, withinserting a sealing member (not shown). In this way, the air-tightnessof the battery is kept. Accordingly, in the case of the battery shown inFIGS. 1A, 1B and 2, the electrolytic solution pouring inlet 14 isactually composed of the inlet 14 and the sealing member, but the inlet14 is illustrated as such without a sealing member in order to make thefigure simple. The explosion-proof vent 15 is made in the cover plate 9.

In the battery 1 of Example 1, the positive electrode lead member 7 isdirectly welded to the cover plate 9, whereby the combination of thebattery case 4 and the cover plate 9 functions as a positive electrodeterminal. The negative electrode lead member 9 is welded to the leadplate 13, and the negative electrode lead member 8 and the terminal 11are made electrically conductive through the lead plate 13, whereby theterminal 11 functions as a negative electrode terminal. However, thefunctions of the positive and negative electrodes may be reversed inaccordance with the material of the battery case 4, etc.

FIG. 2 is a perspective view schematically illustrating the externalappearance of the battery shown in FIGS. 1A and 1B. FIG. 2 shows thatthe above-mentioned battery is a rectangular battery. Thus, FIG. 2schematically shows the battery, and depicts the specific elements outof the constituting elements of the battery.

Example 2

A nonaqueous secondary battery was fabricated in the same manner as inExample 1 except that the positive electrode active material (A) waschanged to LiCo_(0.9988)Mg_(0.0008)Ti_(0.0004)O₂ (average particle size:5 μm), and the positive electrode active material (B) was changed toLiCo_(0.9964)Mg_(0.0024)Ti_(0.0012)O₂ (average particle size: 5 μm) andthe positive electrode active material (C) as changed toLiCo_(0.9960)Mg_(0.0027)Ti_(0.0013)O₂ (average particle size: 1 μm).

The density of the positive electrode mixture layers (the density of thepositive electrode) was 3.95 g/cm³ after the pressing under the samepressure as in Example 1. The thickness of the positive electrodemixture layers (the thickness of the layers on both the surfaces, i.e.,the thickness obtained by subtracting the thickness of the aluminum foillayer of the positive electrode current collector from the totalthickness of the positive electrode) was 130 μm.

Example 3

A nonaqueous secondary battery was fabricated in the same manner as inExample 1 except that the positive electrode active material (A) waschanged to LiCo_(0.998)Mg_(0.0008)Ti_(0.0004)O₂ (average particle size:18 μm), the positive electrode active material (B) was changed to anoxide having the same composition as the positive electrode activematerial (A) and having an average particle size of 5 μm, and thepositive electrode active material (C) was changed to an oxide havingthe same composition as the positive electrode active material (A) andhaving an average particle size of 1 μm.

The density of the positive electrode mixture layers (the density of thepositive electrode) was 3.95 g/cm³ after pressing at the same pressureas in Example 1. The thickness of the positive electrode mixture layers(the thickness of the layers on both the surfaces, i.e., the thicknessobtained by subtracting the thickness of the aluminum foil layer of thepositive electrode current collector from the total thickness of thepositive electrode) was 130 μm.

Example 4

A nonaqueous secondary battery was fabricated in the same manner as inExample 1 except that the positive electrode active material (A) waschanged to LiCo_(0.9988)Mg_(0.0008)Ti_(0.0004)O₂ (average particle size:18 μm), the positive electrode active material (B) was changed to anoxide having the same composition as the positive electrode activematerial (A) and having an average particle size of 5 μm, and thepositive electrode active material (C) was changed to an oxide havingthe same composition as the positive electrode active material (A) andhaving an average particle size of 1 μm.

The density of the positive electrode mixture layers (the density of thepositive electrode) was 3.80 g/cm³ after the pressing at a lowerpressure than in Example 1. The thickness of the positive electrodemixture layers was 135 μm.

Comparative Example 1

A nonaqueous secondary battery was fabricated in the same manner as inExample 1 except that the positive electrode active materials werechanged to a mixture wherein the following were mixed:LiCo_(0.9988)Mg_(0.0008)Ti_(0.0004)O₂ (average particle size: 18 μm) asthe positive electrode active material (A); and an oxide having the samecomposition as the positive electrode active material (A) and having anaverage particle size of 5 μm as the positive electrode active material(B).

The density of the positive electrode mixture layers (the density of thepositive electrode) was 3.70 g/cm³ after the pressing under the sameconditions as in Example 4. The thickness of the positive electrodemixture layers was 138 μm.

In each of Examples 1 to 4 and Comparative Example 1, plural nonaqueoussecondary batteries having the same structures as described above wereprepared, and then their characteristics were evaluated as follows:

Discharge Capacity

Each of the batteries fabricated in Examples 1 to 4 and ComparativeExample 1 was charged at a constant current of 0.2 C up to 4.4 V, andthen charged at a constant voltage until the total charge time reached 8hours. Subsequently, the battery was discharged at a constant current of0.2 C down to a voltage of 3.3 V. At this time, the discharge capacitywas measured. The results are shown in Table 1. In Table 1, thedischarge capacity obtained in each battery is shown as a relative valuein relation to the discharge capacity of the battery of ComparativeExample 1, which is “100”.

Discharge Capacity After Charge-Discharge Cycles

With each of the batteries fabricated in Examples 1 to 4 and ComparativeExample 1, charge and discharge were repeated 50 times under the sameconditions as those in the evaluation of discharge capacities above. Thedischarge capacity in the 50th cycle was used to evaluate the dischargecapacity after the charge-discharge cycles. The results are shown inTable 2. In Table 2, the discharge capacity after the charge-dischargecycles obtained about each of the battery is shown as a relative valuein relation to the discharge capacity of the battery of ComparativeExample 1 after the charge-discharge cycles, which is “100”.

Safety Test

With each of the batteries fabricated in Examples 1 to 4 and ComparativeExample 1, an overcharging test was conducted at 1 C and 12 V. Thehighest temperature (surface temperature) generated by the battery inthe test was measured. The results are shown in Table 3. The lowersurface temperature means the higher safety of the battery in the caseof overcharging.

TABLE 1 Charge voltage: 4.4 V Example No. Discharge Capacity Example 1105 Example 2 104 Example 3 104 Example 4 102 Comparative Example 1 100

TABLE 2 Discharge capacity after Example No. charge-discharge cyclesExample 1 117 Example 2 115 Example 3 110 Example 4 104 ComparativeExample 1 100

TABLE 3 Example No. Highest temp. ((C.) Example 1 121 Example 2 130Example 3 125 Example 4 131 Comparative Example 1 131

The nonaqueous secondary battery of Comparative Example 1, which had thepositive electrode comprising two lithium-containing transition metaloxides having different average particle sizes as the positive electrodeactive materials, had the lower density of the positive electrodemixture layers than the nonaqueous secondary batteries of Examples 1 to4 according to the present invention. This is also apparent from thefact that the positive electrode mixture layers in Comparative Example 1had a lower density than that of the positive electrode mixture layersin Example 4, which was formed under the same pressing pressure as inComparative Example 1. It is also seen from the results of Table 1 thatthe battery of Comparative Example 1 had a smaller capacity by about 2to 5% than the batteries of Examples 1 to 4. Thus, the nonaqueoussecondary batteries of Examples 1 to 4, which had a positive electrodecontaining, as its positive electrode active materials, threelithium-containing transition metal oxides having different averageparticle sizes, achieved the high capacity.

In the nonaqueous secondary battery of Example 1, the highesttemperature generated in the safety test was lower than that generatedby the nonaqueous secondary battery of Comparative Example 1. Thus, theformer battery has better safety than the latter. Furthermore, thenonaqueous secondary batteries of Examples 1 to 4 also exhibited a highdischarge capacity after the charge-discharge cycles, and thus had agood charge-discharge cycle characteristics. In the case that theadditive element, aluminum, is not contained as in Example 2, thehighest temperature generated by the battery was higher than that inExample 1. Thus, it is understood that the safety can be increased bythe addition of aluminum.

In the nonaqueous secondary battery of Example 1, in which the contentof the metal element M² in the positive electrode active material (C)having the smallest average particle size is larger than that in thepositive electrode active material (A) or (B), the safety (the highesttemperature generated by the battery exhibits) and the charge-dischargecycle characteristics are better than in the nonaqueous secondarybattery of Example 3, in which the content of the metal element M² wasthe same among the positive electrode active materials (A), (B) and (C).

The invention claimed is:
 1. A nonaqueous secondary battery comprising:a positive electrode having a positive electrode mixture layer, anegative electrode, and a nonaqueous electrolyte, wherein the positiveelectrode comprises an active material consisting of at least threelithium-containing transition metal oxides having different averageparticle sizes and at least five inflection points in a particle sizedistribution curve of the mixture of said at least threelithium-containing transition metal oxides, wherein thelithium-containing transition metal oxides comprise at least onetransition metal element M¹ selected from the group consisting of Co, Niand Mn, and the transition metal element M¹ of the lithium-containingtransition metal oxide having the smallest average particle size ispartially substituted with a metal element M² other than the transitionmetal element M¹, wherein the transition metal element M² comprises Mgand at least one metal element selected from the group consisting of Ti,Zr, Ge, Nb, Al and Sn, wherein a content of the transition metal elementM² in the lithium-containing transition metal oxide having the smallestaverage particle size is largest among said at least threelithium-containing transition metal oxides.
 2. The nonaqueous secondarybattery according to claim 1, wherein said transition metal element M¹of at least one of the lithium-containing transition metal oxides otherthan one having the smallest average particle size is partiallysubstituted with a metal element M² other than the transition metalelement M¹.
 3. A method of using a nonaqueous secondary batteryaccording to any one of claims 1 to 2 comprising the step of: chargingthe battery so that a positive electrode voltage is in a range of 4.35to 4.6 V with reference to the potential of lithium when the battery isfully charged.
 4. The aqueous secondary battery according to claim 1,wherein the particle size of the lithium-containing transition metaloxide having the smallest average particle size is 0.5 to 4 μm.
 5. Theaqueous secondary battery according to claim 1, wherein the particlesize of the lithium-containing transition metal oxide having the secondlargest average particle size is 2 to 10 μm.
 6. The aqueous secondarybattery according to claim 1, wherein the particle size of thelithium-containing transition metal oxide having the smallest averageparticle size is 0.5 to 4 μm, and the particle size of thelithium-containing transition metal oxide having the second largestaverage particle size is 2 to 10 μm.
 7. The nonaqueous secondary batteryaccording to claim 1, wherein the positive electrode comprises an activematerial consisting of three lithium-containing transition metal oxideshaving different average particle sizes.
 8. The nonaqueous secondarybattery according to claim 1, wherein a thickness obtained bysubtracting a thickness of an electrode current collector from a totalthickness of the positive electrode is from 30 to 200 μm.
 9. Thenonaqueous secondary battery according to claim 1, wherein a particlesize of the lithium-containing transition metal oxide having the largestaverage particle size is 5 to 25 μm.
 10. The nonaqueous secondarybattery according to claim 1, wherein the density of the positiveelectrode mixture layer is at least 3.8 g/cm³.